We have plans to pursue three main objectives: (1)\To perfect techniques for injecting cell balls and for making trophoblastic vesicles by using real inner cell mass cells for injection. Since we know these cells can develop and can replace inner cell masses, their success in doing so in any particular experiment will be a measure of the quality of our technique. Methodology will be as described before. (2)\To use the techniques we develop to inject inner-cell-mass-like balls of cells from a variety of teratocarcinoma lines into trophoblastic vesicles. If they fail to develop in situations where injected normal inner cell mass cells do develop, then we can say that the teratocarcinoma cells are not equivalent to and cannot replace inner cell mass cells. (3)\To attempt to repeat the finding of Brinster, et al., that sperm can carry DNA, with which they have been incubated, into eggs at the time of fertilization. We will attempt to make mouse sperm carry human HPRT DNA into mouse eggs at the time of fertilization. We will look for human HPRT enzyme and human DNA in the mouse cells on the day after fertilization, 8-10 days after fertilization and at birth (and later if these early times are positive).